The melting and recrystallization of silicon blocks (ingots) from silicon particles, silicon granules or silicon pieces are carried out using crucibles made of graphite or silicon nitride, but mainly SiO2 (fused silica). Ingots having the desired microstructures and purities crystallize from the melt during precisely defined cooling processes, and these ingots are subsequently cut into thin wafers and form the active constituent of photovoltaic units.
It is important here that the solar silicon quality is not adversely affected by the materials used in processing, e.g. melting crucibles, and the silicon melt can solidify without defects and can be removed undamaged from the crucible. In this context, it is important to prevent corrosive attack of liquid silicon metal on the crucible material, since the melt would otherwise become contaminated. Furthermore, adhesion, infiltration and diffusion lead to problems in the demoulding of the ingots, so that there is a risk of rupture or cracking of the polycrystalline silicon block.
The corrosive silicon melt results in attack on the SiO2 crucible, since a chemical reaction between Si and SiO2 takes place to form volatile SiO. In addition, oxygen and undesirable impurities from the crucible get into the silicon melt in this way.
In particular, adhering material on the solidifying or solidified silicon block is to be avoided at all costs, since silicon undergoes very large thermal expansions so that very small amounts of adhering material lead to mechanical stress and thus to fracture of the crystalline structure, which results in reject silicon material.
In aluminium metallurgy, in particular in low-pressure aluminium casting, riser tubes made of iron alloys or fused silica are used. Due to the highly corrosive aluminium melt at temperatures in the range from 650 to 800° C., these riser tubes have to be coated with refractory oxides or nitrides at regular intervals in order to avoid excessively rapid corrosion of these materials by the liquid aluminium. In the case of iron alloys, the corrosion mechanism is a dissolution mechanism, while in the case of fused silica silicon dioxide reacts with the molten aluminium according to the reaction equation:3 SiO2+4 Al-->2 Al2O3+3 Si.
Adhering reaction products and melt residues are formed on the fused silica riser tube and different coefficients of expansion between riser tube and corrosion products lead to premature failure of the riser tube. Use is usually made here of coatings of aluminium oxide or boron nitride which are applied from slips containing organic binders by dipping, brushing or spraying. However, as a result of the combined corrosive and mechanical attack by the hot melt and the floating slag, the life of such coatings is limited to hours or a few days. Riser tubes made of silicon nitride ceramic, which are completely inert towards corrosive attack by aluminium melts, are also used as alternatives to the coated riser tubes made of iron alloy or fused silica. However, the costs of these silicon nitride tubes are many times that of standard riser tubes having a coating.